Rett syndrome is a neurodegenerative disorder affecting females only. There are no known cases of affected males. Familial cases are rare but have been reported in half sisters and maternal cousins. Very little is known about the molecular genetics and pathogenesis of this disorder. To date laboratory investigations have not provided any clues to help understand the pathogenesis and biology of this disorder. The approach proposed here is aimed at understanding the genetic basis of Rett syndrome, and accurately mapping the gene with the ultimate goal of cloning the Rett syndrome cDNA and genomic sequence. This project can be achieved by the use of the most recent advances in recombinant DNA technology. As a first step in understanding the genetics of this disorder, experiments designed to test for nonrandom X inactivation in Rett syndrome will be performed. These experiments will rely on the use of recombinant DNA probes that simultaneoulsy detect restriction fragment length polymorphisms and patterns of methylation of X chromosome genes. Three different X chromosome probes can be used for such analysis; the hypoxanthine phosphoribosyl transferase gene, the phosphoglycerate kinase gene, and sequences defined by the clone pXUT23. The first hypothesis to be tested is whether the chromosome carrying the Rett gene is active in Rett patients using skin fibroblast and peripheral blood leukocytes. The second hypothesis aimed at evaluating whether the good X chromosome with the normal gene is selectively active in mothers of Rett patients in the familial cases. Human/hamster hybrids to separate the two X chromosomes and identify the active human X will be prepared so that X inactivation patterns can be studied when the above DNA probes fail to detect polymorphisms. A detailed search for submicroscopic deletions of the X chromosome will be carried out through the use of restriction fragment length polymorphisms and the somatic cell hybrids containing each of the X chromosomes of the Rett patients. Regional mapping of the Rett syndrome gene will be attempted through the use of highly polymorphic X chromosome DNA probes and DNA from patients known to belong to the familial cases. After the above is achieved, longer term goals will focus on cloning the Rett gene.